\begin{tikzpicture}[>=Stealth, scale=0.8]
    \draw [->] (-1, 0) -- (4.5, 0) node[anchor=west] {$x$};
    \draw [->] (0, -1) -- (0, 4) node[anchor=east] {$y$};
    \node at (-0.3, -0.3) {$O$};

    \coordinate (O) at (0, 0);
    \coordinate [label=100:$Z_1$] (Z1) at (2.2, 0.5);
    \coordinate [label=90:$Z_2$] (Z2) at (1, 2);
    \coordinate [label=45:$Z$] (Z) at (3.5, 2.8);
    \draw[->, thick] (O) -- (Z1);
    \draw[->, thick] (O) -- (Z2);
    \draw[->, thick] (O) -- (Z);
    \draw (Z1) -- (Z) -- (Z2);

    \coordinate [label=270:$Q$] (Q) at (1, 0);
    \coordinate [label=270:$P$] (P) at (2.2, 0);
    \coordinate [label=270:$R$] (R) at (3.5, 0);
    \coordinate [label=0:$S$] (S) at (3.5, 0.5);
    \draw[dashed] (Q) -- (Z2);
    \draw[dashed] (P) -- (Z1) -- (S);
    \draw[dashed] (R) -- (Z);
\end{tikzpicture}
